Awesome point, so then how would one generate a uri relative to the root? Or, does the uri spec leave this unanswered.
UPDATE: Thanks to the above post reference, I'm able to answer my own question in the same section

For example, the URL <URL:ftp://myname@host.dom/%2Fetc/motd> is
interpreted by FTP-ing to "host.dom", logging in as "myname"
(prompting for a password if it is asked for), and then executing
"CWD /etc" and then "RETR motd". This has a different meaning from
<URL:ftp://myname@host.dom/etc/motd> which would "CWD etc" and then
"RETR motd"; the initial "CWD" might be executed relative to the
default directory for "myname". On the other hand,
<URL:ftp://myname@host.dom//etc/motd>, would "CWD " with a null
argument, then "CWD etc", and then "RETR motd".

So the answer is, relative to the drop is default (ftp-host/), relative to the root is ftp-host/%2F